Current Federal and State legislation generally requires control of vehicle exhaust emissions. Oxides of Nitrogen (“NOx”) are one of the exhaust gas emissions that must be controlled. Formation of NOx typically occurs at higher combustion temperatures. A system, generally referred to as the exhaust gas recirculation (“EGR”) system, has been developed to reduce peak combustion temperatures that reduce NOx emissions. An illustrative schematic of this system is generally shown in FIG. 1.
In this type of system, a portion of the exhaust gas is recirculated back to the intake manifold where it is combined with incoming charged air. When this mixture is compressed and ignited in the cylinder, the result is a lower combustion temperature and a reduction in NOx. The system typically consists of an EGR valve (1) that controls the flow of exhaust gas to the intake manifold. Conduits (2), (3), and (4) provide the interconnection between the exhaust manifold (5), EGR valve (1), and intake manifold (6). The system shown uses an electrically controlled EGR valve. An engine control unit (“ECU”) (7) provides a signal that controls the open and/or closing of the valve. As the EGR valve (1) opens and closes, it will increase or decrease the flow rate of exhaust gas to the intake manifold (6). It is also typical to have a throttle valve (8) to control airflow into the intake manifold and, an exhaust gas cooler (9) to reduce temperature of recirculated exhaust gas.
The required EGR flow rate is dependent upon several factors that include the displacement of the engine and the pressure differential between the exhaust and the intake system. EGR valves may be actuated by pneumatic or electric means. Pneumatically actuated valves depend upon the availability of pressure or vacuum on the vehicle and this may be an undesirable requirement. They also require a means of electrically controlling the pneumatic source to allow overall electrical control of the system. An electric vacuum or pressure regulator is use to provide this control.
Operating force is another factor used in the selection criteria for the type of actuator used for the EGR valve. Higher flow rates require larger valves with greater area and higher operating forces. Lower pressure differential between the exhaust and intake manifold will require larger valves to achieve the desired flow rate. Components in the exhaust gas can accumulate on the valve components and cause them to stick or restrict movement if sufficient operating force is not available.
Referring to FIG. 2, a conventional EGR valve typically includes an actuator/valve assembly (10) and a valve base assembly (20). The EGR valve is typically mounted by fastening the valve body (20A) of the valve body assembly (20) to the intake manifold of the engine. A gasket is typically used as a seal to prevent leakage of exhaust gas to the environment. A valve poppet (30) is installed and retained on a valve stem (40) by any number of suitable methods, such as but not limited to radial riveting. The poppet valve (30) can be keyed to the shaft in a manner that will cause it to rotate with the shaft.
Still referring to FIG. 2, the valve base assembly (20) typically includes a valve seat (50) that is secured by suitable methods such as but not limited to press fit and/or staking. The actuator/valve assembly (10) and the valve body assembly (20) are combined to form the EGR valve. Fasteners (55) are used to secure the two assemblies together. Suitable locating features, in the actuator/valve assembly (10) and valve body assembly (20), are used to align the valve poppet (30) and valve seat (50) such that suitable sealing is provided when the valve poppet (30) is seated on the valve seat (50).
Referring to FIGS. 2 and 3, the EGR valve typically operates in the following manner. The ECU applies an electrical control signal to the actuator/valve assembly (10) that causes the valve poppet (30) to lift off of the valve seat (50). When there is a sufficient pressure differential between the inlet and outlet, the exhaust gas will flow through the EGR valve. The exhaust gas will flow from the inlet (60), into the chamber (70), through the valve seat (50), by the valve poppet (30), into the cavity (80), and to outlet (90). It should be appreciated the EGR system shown employs an EGR cooler that is operable to cool the exhaust gas prior to the exhaust gas reaching the EGR valve.
Components in the exhaust gas can accumulate on the valve components and cause them to stick or restrict movement if sufficient operating force is not available. By way of a non-limiting example, during normal operation of diesel engines, especially those employing cooled EGR systems, the EGR valve poppet often becomes stuck to the valve seat in the closed position, due to excessive build up of various exhaust gas components, which renders the valve inoperable.
More specifically, certain EGR systems that run with cooled exhaust gas (e.g., cooled or cold side EGR systems) may have a tendency to produce a moist vapor like (e.g., lacquer) material, until the engine warms up, which builds up on the valve poppet (30) and valve seat (50) as exhaust gas flows past them, as previously described. This material could combine with a powdery (e.g., soot) type of contamination that is present in the exhaust gas at elevated exhaust gas temperatures (e.g., greater than 140° C.). When the EGR valve is commanded to the closed position, the lacquer, soot or a combination of the two, starts to harden and causes a “bond” to be formed between the valve seat and poppet. This often happens after then engine is shut down for a time duration of about 20 minutes or greater. When the engine is started again, and the EGR valve is commanded to open, the “bond” that has formed prevents the EGR valve from opening when there is insufficient force and or torque available from the EGR valve to overcome the bonded sticking force.
Accordingly, there exists a need for new and improved EGR valve systems that are able to avoid sticking and/or bonding of the various surfaces of the components thereof, especially the valve poppet and valve seat surfaces.